SUMMARY Exploring the Impact of Human Milk Oligosaccharides in Atherosclerosis. Cardiovascular diseases (CVD) are the leading cause of death globally for both men and women. The majority of CVD-related deaths are primarily caused by complications of atherosclerosis, a chronic inflammatory disorder. The key role for inflammation in all stages of atherosclerosis has shifted the attention to targeted therapies preventing further infiltration of pro-inflammatory macrophages and production of their cytokines. The draw back of such therapeutic strategies is that they can influence LDL cholesterol, have adverse effects, require administration via injections and have high costs. Human milk oligosaccharides (HMOs) are a group of structurally diverse unconjugated glycans that are highly abundant in and unique to human milk, the optimal and natural source of nutrition for the human infant. Unlike most oligosaccharides, HMOs resist the low pH in the stomach as well as digestion by gastro-intestinal enzymes and can be detected in significant amounts in the infant's circulation. It is well established that HMOs have beneficial immunomodulatory properties by either acting on a cellular or on a systemic level. Our extensive preliminary data identified one specific HMO called 3'sialyllactose (3'SL) that was highly effective in attenuating M1 macrophage activation and reduced pro-inflammatory cytokine expression in murine and human macrophages. We will test the hypothesis that 3'SL can attenuate inflammation and atherosclerosis development in an adult setting. Towards this goal we will (i) investigate the underlying mechanisms of how 3'SL attenuates macrophage and endothelial inflammation relevant for atherosclerosis and (ii) assess in vivo efficacy in a mouse model of atherosclerosis by examining early- and late-stage atherogenesis of Ldlr-/- mice on a high-fat/high cholesterol diet treated with or without 3'SL and a control HMO. Knowledge gained from the proposed project will provide insights into the mechanisms of how 3'SL modulates immune cell responses and guide the development of 3'SL, a natural compound from human milk that is considered safe for human consumption, as a new therapeutic that can be used in the prevention of fatal CVD events and potentially also of other chronic inflammatory disorders in adults such as rheumatoid arthritis and obesity.